The present invention relates to an image reading device, and more specifically to an image reading device such as a scanner utilized in an electronic copier for example and other image forming apparatuses.
Conventionally, in an image reading device such as a scanner utilized in an electronic copier for example and other image forming apparatuses, a document placed on a document table made of a transparent sheet of glass is illuminated by an illumination lamp, and the reflected light is captured by a CCD line sensor for photoelectric transformation, whereby reading of a document image is performed.
The illuminating lamp and a mirror for guiding the reflected light from the document to the CCD line sensor are provided on a scanning carriage.
With such an arrangement in the image reading device such as the scanner, when reading the document placed on the document table, the document is read line by line in main scanning directions while the scanning carriage is moved in a sub-scanning direction, whereby reading of an entire image of the document is performed.
Further, conventionally, in an image reading device (hereinafter called a scanner) provided with an automatic document feeder (hereinafter called an ADF), when reading a manually placed document which is manually put on the glass serving as the document table, the scanning carriage is moved in the sub-scanning direction as described above, whereas the scanning carriage is held stationary during the reading operation in which the document is fed by the ADF.
In the above arrangement, reading of the document fed by the ADF and the reading of the document manually placed on the table are performed at the same speed.
For example, suppose that the document is to be read at a magnification rate of 100% to compare the ADF feeding mode and reading in a manual mode. The speed of the roller which feeds the document in the ADF feeding and the moving speed of the scanning carriage relative to the placed document are the same.
However, according to the above method, when transporting the document by the ADF, one component is driven at a certain speed, and when transporting the scanning carriage, another component is driven at the same speed. Such an arrangement tends to place a demanding specification on both the components.
Therefore, unwanted limitations are placed on certain conditions, such as specifications for a roller driving motor and a scanning carriage driving motor, or certain requirements call for the use of an expensive part or member.
Take, for example, a high speed scanner utilized in a 65 cpm class digital copier (PPC).
In order to achieve the 65 cpm capability in the ADF document reading, a total amount of time of a document transporting time and a document holding time for all of the 65 sheets of the document must be within a range of one minute.
Moreover, the document holding time is not something that may be given perfunctorily, but must allow for enough time necessary for data processing. For this reason, the document holding time cannot be made shorter than a certain amount of time.
So, after subtracting the total document holding time, there is left only a very limited amount of time usable for transporting the 65 sheets of document, which means that the transporting speed must be significantly high.
Likewise, when reading the manually placed document, if the reading magnification is the same as in the ADF document reading, then the scanning carriage will be driven at the same driving speed as in the above transportation speed.
However, if the scanning carriage is to be moved at a high speed within a short time in the manual document reading, there is a concern for deterioration in image quality due to vibration, because the vibration generated during acceleration of the scanning carriage will be still present when the image reading operation is started.
In order to reduce the vibration, an expensive motor has to be used as the scanning carriage driving motor, or a rigid anti-vibration structure must be provided for the scanning carriage as well as providing a travel distance long enough for the scanning carriage.
In order to provide such a sufficient distance for the acceleration of the scanning carriage, the scanner unit must be made bigger, for example.